The present invention relates to a controlling method of a seat belt retractor, i.e. motorized seat belt retractor, which can change its operating condition to correspond to the state of an occupant wearing a seat belt or to external signals.
In a seat belt retractor provided in an automobile, it is preferable to retract an excess amount of a seat belt after the seat belt is pulled and a tongue is engaged with a buckle device, without applying too much stress to the chest or other portions of an occupant normally wearing the seat belt.
Normally used for winding up a seat belt in a seat belt retractor is a biasing force of a single return spring. When a spring providing a small biasing force is employed in order to reduce stress applied to the chest of an occupant, the force for winding or retracting the seat belt becomes weak, thus lowering its operation and its accommodation to the retracted state.
To the contrary, when a spring for providing a large biasing force is employed in order to provide an enough winding force for its retraction, the stress to be applied to the chest of the occupant normally wearing the seat belt is increased.
Since a single return spring is employed, a conventional seat belt retractor has a problem that the biasing force of the return spring increases as the amount of withdrawal of the webbing (hereinafter, the term “webbing” is used for referring to the belt itself in this specification) is increased because the return spring is repeatedly wound.
As one of means for solving the problem with regard to the winding of a webbing caused by using a single return spring, the applicant has developed a tension reducing mechanism for reducing the retraction force, which comprises two rope pulleys of truncated cone shape with a helical guide groove (see Japanese Patent No. 2711428).
On the other hand, the applicant also has developed a seat belt retractor in which a tension control for a seat belt is performed by a built-in electric motor to improve comfortableness as one of functions when an occupant normally wears, and to correspond to a distance from another vehicle running in front of or behind its own vehicle (see Japanese Unexamined Patent Publication No. H09-132113).
In the seat belt retractor disclosed in the aforementioned publication No. H09-132113, an ultrasonic motor is employed as a driving unit for the tension control and a spring mechanism is provided as a main winding mechanism. The motor is employed for controlling the tension on the seat belt when the occupant wears the seat belt, and for controlling the winding and unwinding of the seat belt to correspond to external signals inputted during running of the vehicle. Therefore, it is required to rotate the motor in reverse. This makes the control circuit complex. Since the operation of winding up the seat belt is performed by two circuits, there is also a problem that it is hard to smoothly switch the operation of winding up the seat belt according to the inputted external signals.
When the external signals from sensors mounted on the vehicle are set to function as the trigger of the motor, a speed-reduction gear to be driven by the motor employed in the seat belt retractor can not cope with a wide range of a winding mode because the reduction range allowed by the speed-reduction gear is limited.
Therefore, it is an object of the present invention to solve the problems of the prior art and to provide a seat belt retractor in which a seat belt can be wound by only a motor with a simple control circuit and a driving mechanism, and the winding mode can be set widely to correspond to external signals inputted when an occupant wears the seat belt and a vehicle runs, and to provide a control method for the seat belt retractor.
Further objects and advantages of the invention will be apparent from the following description of the invention.